This invention relates to vinyl-containing polysilanes of the general formula EQU [RSi][R.sub.2 Si]
where there are present 0 to 60 mole percent of [R.sub.2 Si] units and 40 to 100 mole percent of [RSi] units, and to vinyl-containing polysilanes of the general formula EQU [Rsi][R.sub.2 Si][R"Si]
where there are present 0 to 40 mole percent [R.sub.2 Si] units, 0.1 to 99.9 mole percent [RSi] units, and 0.1 to 99.9 mole percent [R"Si] units, where R is an alkyl radical containing 1 to 8 carbon atoms, where R" is selected from the group consisting of alkyl radicals containing at least six carbon atoms, phenyl radicals, and radicals of the formula A.sub.y X.sub.(3-y) Si(CH.sub.2).sub.z -- where A is a hydrogen atom or an alkyl radical containing 1 to 4 carbon atoms, y is an integer equal to 0 to 3, X is chlorine or bromine, and z is an integer greater than or equal to 1, where the remaining bonds on silicon are attached to other silicon atoms, --R' groups, and vinyl radicals, where R' is an alkyl group containing 1 to 8 carbon atoms or a phenyl radical, where the --R' and vinyl groups are endblocking groups, and where the --R' groups are attached both to the most reactive and the least reactive endblocking sites and the vinyl groups are attached to the endblocking sites of intermediate reactivity. These vinyl-containing polysilanes are prepared by reacting a chlorine- or bromine-endblocked polysilane of general formula EQU [RSi]{R.sub.2 Si],
where there are present 0 to 60 mole percent [R.sub.2 Si] units and 40 to 100 mole percent [RSi] units, or a chlorine- or bromine endblocked polysilane of general formula EQU [RSi][R.sub.2 Si][R"Si]
where there are present 0 to 40 mole percent [R.sub.2 Si] units, 0.1 to 99.9 mole percent [RSi] units, and 0.1 to 99.9 mole percent [R"Si] units where R is an alkyl radical containing 1 to 8 carbon atoms, where R" is selected from the group consisting of alkyl radicals containing at least six carbon atoms, phenyl radicals, and radicals of the formula A.sub.y X.sub.(3-y) Si(CH.sub.2).sub.z -- where A is a hydrogen atom or an alkyl radical containing 1 to 4 carbon atoms, y is an integer equal to 0 to 3, X is chlorine or bromine, and z is an integer greater than or equal to 1, and where the remaining bonds on silicon are attached to other silicon atoms and bromine or chlorine atoms, with, first, a Grignard reagent of general formula R'MgX' or an organolithium compound of general formula R'Li, followed with, second, a vinyl-containing Grignard reagent of general formula (CH.sub.2 .dbd.CH)MgX' or vinyllithium, followed with, third, a Grignard reagent of general formula R'MgX' or an organolithium reagent of general formula R'Li under carefully controlled reaction and process conditions where R' is an alkyl radical with 1 to 8 carbon atoms or a phenyl radical and X' is chlorine, bromine, or iodine whereby both the most reactive and the least reactive endblocking sites are occupied by --R' groups and the endblocking sites of intermediate reactivity are occupied by vinyl groups. The reaction conditions must be carefully controlled to ensure that the vinyl groups, which are incorporated into the polysilane via a derivatization reaction, survive the reaction and processing steps intact. The order of the derivatization reactions, whereby the chlorine- or bromine-endblocking groups are replaced by --R' and vinyl endblocking groups, is used to ensure that the vinyl groups occupy endblocking sites of intermediate reactivity.
This invention also relates to a method of preparing such polysilanes under carefully controlled conditions to ensure the vinyl groups survive the reaction process and occupy endblocking sites of intermediate reactivity. This invention further relates to the silicon carbide ceramics prepared from such vinyl-containing polysilanes. The vinyl-containing polysilanes of this invention may be rendered infusible by exposure to UV irradiation in an inert atmosphere prior to pyrolysis to form ceramic material. Such cure mechanisms can result in ceramic materials containing only limited amounts of oxygen. The vinyl-containing polysilanes of this invention can also be cured in oxygen containing atmospheres but the resulting ceramic material obtained from such air cured polymers will contain increased amounts of oxygen.
Ceramic fibers prepared from the vinyl-containing polysilanes of this invention have very high tensile strengths. Furthermore, the stiochiometry of silicon and carbon in the ceramic material can be readily controlled by variations in the amounts of --R' and vinyl groups in the vinyl-containing polysilane.
Haluska in U.S. Pat. Nos. 4,546,163 (issued Oct. 8, 1985) and 4,595,472 (issued June 17, 1986) claimed to produce vinyl-containing polysilanes by a redistribution mechanism by reacting various disilanes and vinyl-containing silanes in the presence of a redistribution catalyst. More careful work has now determined that the vinyl group itself is not incorporated into the resulting polysilane under the reaction conditions employed. The polysilanes of Haluska from the redistribution reaction of disilanes and vinyl silanes do not contain vinyl groups. This is illustrated in Comparative Example 1 infra.
Haluska in the just mentioned patents also claimed that the vinyl content of his "vinyl-containing" polysilanes could be increased by reacting the "vinyl-containing" polysilane with a vinyl Grignard reagent or vinyllithium. However, it has now been determined that under the conditions Haluska employed during isolation of the end product (i.e. temperatures between 200.degree. and 250.degree. C.) that the vinyl groups will not survive. This is illustrated in Comparative Example 2 infra.
Based on these observations, it is clear that the alleged "vinyl-containing" polysilanes of U.S. Pat. Nos. 4,546,163 and 4,595,472 do not contain vinyl groups. In the examples of both patents, the vinyl content was merely calculated based on the initial reactants and the analyzed by-products using the assumption that any unaccounted vinyl groups must have been incorporated into the polymer. The vinyl content was not determined experimentally. As indicated in Comparative Example 1 infra, NMR analysis confirms the absence of vinyl groups in polysilanes prepared by the Haluska method.
This present invention differs from that of Haluska in that the reaction and process conditions under which a chlorine or bromine endblocked polysilane and a vinyl Grignard reagent or vinyllithium are reacted are carefully controlled to ensure the survival of the vinyl groups in the resulting polysilane. The present invention results in vinyl-containing polysilanes which are useful in preparing ceramic materials. The polysilanes of Haluska lack the desired vinyl groups. The presence of vinyl groups in the polysilanes of the present invention are confirmed by NMR analysis. Furthermore, in the present invention the vinyl endblocking groups occupy endblocking sites of specific reactivity. This allows these vinyl-containing polysilanes to have a softening temperature below the temperature that the vinyl groups will thermally crosslink. The vinyl-containing polysilanes of this invention can be melt spun to form fibers, cured by UV irradiation, and then pyrolyzed to yield ceramic fibers.
Bujalski, et al. in a copending application entitled "A Method of Producing Silicon Carbide Preceramic Polymers," which was filed the same day as this application and which is hereby incorporated by reference, disclosed a method of preparing vinyl-containing polysilanes by reaction of a chlorine- or bromine-endblocked polysilane with (CH.sub.2 .dbd.CH)MgX or (CH.sub.2 .dbd.CH)Li. These vinyl-containing polysilanes have softening points above the temperature at which the vinyl crosslinking reaction becomes dominant. Thus these vinyl-containing polysilanes will cure before they melt; this makes it very difficult to melt spin these polysilanes. In the present invention, the softening temperature has been reduced below the crosslinking temperature of the vinyl groups by controlling the reactivity of endblocking sites occupied by the vinyl groups. Therefore, the vinyl-containing polysilanes of the present invention can be easily formed into fibers by melt spinning, cured by UV irradiation, and then converted into ceramic fibers by pyrolysis.
Baney et al. in U.S. Pat. No. 4,310,651 (issued Jan. 12, 1982) disclosed a polysilane of general formula EQU [CH.sub.3 Si][(CH.sub.3).sub.2 Si]
where there was present 0 to 60 mole percent [(CH.sub.3).sub.2 Si] units and 40 to 100 mole percent [CH.sub.3 Si] units and where the remaining bonds on silicon were attached to other silicon atoms and chlorine atoms or bromine atoms. The polysilane was converted to a beta-silicon carbide containing ceramic at elevated temperatures (about 1400.degree. C.). The polysilanes of U.S. Pat. No. 4,310,651 generally are difficult to handle due to their high reactivity in air.
Baney et al. in U.S. Patent 4,298,559 (issued Nov. 3, 1981) prepared polysilanes of general formula EQU [CH.sub.3 Si][(CH.sub.3).sub.2 Si]
where there was present 0 to 60 mole percent [(CH.sub.3).sub.2 Si] units and 40 to 100 mole percent {CH.sub.3 Si] units and where the remaining bonds on silicon were attached to other silicon atoms and additional alkyl radicals of 1 to 4 carbon atoms or phenyl radicals. Upon heating, these polysilanes were converted into silicon carbide containing ceramics in high yields.
Baney et al. in U.S. Patent Re. No. 31,447 (reissued Nov. 22, 1983) disclosed polysilanes of the general formula EQU [CH.sub.3 Si][(CH.sub.3).sub.2 Si]
where there was present 0 to 60 mole percent [(CH.sub.3).sub.2 Si] units and 40 to 100 mole percent [CH.sub.3 Si] units and where the remaining bonds on silicon were attached to other silicon atoms and alkoxy radicals containing 1 to 4 carbon atoms or phenoxy radicals. Silicon carbide ceramics were obtained by firing these polysilanes to elevated temperatures.
Baney et al. in U.S. Pat. No. 4,314,956 (issued Feb. 9, 1982) disclosed polysilanes of the general formula EQU [CH.sub.3 Si][(CH.sub.3).sub.2 Si]
where there was present 0 to 60 mole percent [(CH.sub.3).sub.2 Si] units and 40 to 100 mole percent [CH.sub.3 Si] units and where the remaining bonds on silicon were attached to silicon and amine radicals of the general formula --NHR.sup.vi where R.sup.vi is a hydrogen atom, an alkyl radical of 1 to 4 carbon atoms or a phenyl radical. A silicon carbide ceramic was obtained by firing this polysilane to an elevated temperature under an inert atmosphere or under an ammonia atmosphere.
The just discussed U.S. Pat. Nos. 4,310,651, 4,298,599, Re. No. 31,447, and 4,314,956 are hereby incorporated by reference. These polysilanes are further discussed in Baney et al. Organometallics, 2, 859 (1983).
West in U.S. Pat. No. 4,260,780 (issued Apr. 7, 1981) prepared a polysilane of general formula EQU [(CH.sub.3).sub.2 Si][CH.sub.3 (C.sub.6 H.sub.5)Si]
by the sodium metal reduction of dimethyldichlorosilane and methylphenyldichlorosilane. The resulting polysilanes had very high softening points (&gt;280.degree. C.).
West et al. in Polym. Prepr., 25, 4 (1984) disclosed the preparation of a polysilane of general formula EQU [CH.sub.3 (CH.sub.2 .dbd.CHCH.sub.2)Si][CH.sub.3 (C.sub.6 H.sub.5)Si]
by the sodium metal reduction of allylmethyldichlorosilane and methylphenyldichlorosilane. These polysilanes were rapidly gelled by irradiation with ultraviolet light.
Seyferth et al. in U.S. Pat. No. 4,639,501 (issued Jan. 27, 1987) prepared preceramic polymers by reacting a methylpolysilane of the general formula [(RSiH).sub.x (RSi).sub.y ].sub.n with an organosilicon compound having at least two vinyl groups of the general formula [R.sub.2 (CH.sub.2 .dbd.CH)Si].sub.2 Y, where, for example, Y is O, S, NH, NR, or is absent, using either UV irradiation, thermal energy, or catalysts.
It has now been determined that polysilanes of the general formula EQU [RSi][R.sub.2 Si]
or of the general formula EQU [RSi][R.sub.2 Si][R"Si]
which contain vinyl groups as endblocking groups in sites of intermediate reactivity may be prepared in good yield. The presence of vinyl groups in the polysilanes is confirmed experimentally. These polysilane may be pyrolyzed at elevated temperatures in an inert atmosphere to produce silicon carbide-containing ceramics. The polysilanes may be cured by exposure to UV irradiation prior to the pyrolysis step. These polysilanes may also be melt spun, cured by UV irradiation, and pyrolyzed at elevated temperatures in an inert atmosphere to produce silicon carbide-containing ceramic fibers which have very high tensile strengths.